CLASS- 7 SCIENCE (Medium-eng)

CHAPTER- 4 ACIDS, BASESES AND SALTS 

NOTES

1. Acids- Substances that have a sour taste are called acids. The chemical nature of such substances is acidic. Examples include yogurt, lemon juice, orange juice, etc.

2. Bases- Substances that have a bitter taste and feel soapy when touched are called alkalis. The nature of these substances is called Base.

3. Indicators- Special types of substances are used to test whether a substance is acidic or alkaline. These are called indicators. Examples include turmeric, litmus, hibiscus petals, etc.

4. Indicators in Our Surrounding Nature-

1) Litmus- A commonly used natural indicator is litmus, which is extracted from lichens. It is available in the form of a solution or as paper strips, known as litmus paper. It is available in red and blue litmus paper forms. Acids turn blue litmus paper red, and alkalis turn red litmus paper blue. Solutions that do not change the color of red or blue litmus paper are called neutral solutions.

2) Turmeric- Turmeric is used in the form of turmeric paper, which is prepared by applying it to a solution or filter paper and drying it. When a drop of an alkali is placed on this paper, it turns red. However, it does not react with acids. When acid is applied to the red turmeric paper, it restores it to its original state.

3) Hibiscus (China rose)- Place some petals of hibiscus flowers in a beaker and add warm water. After some time, the mixture turns colorful. This solution turns pink with acidic solutions and green with alkaline solutions.

5. Neutralization- 

Phenolphthalein- It gives a pink color with alkaline solutions, while it remains colorless with acidic solutions.

• When an acidic solution is mixed with an alkaline solution, both solutions neutralize each other's effects. The nature of both is eliminated.

• Salt- In the neutralization reaction, a new substance is formed, which is called salt.

• The reaction between an acid and a base is called neutralization. In this process, heat is released along with the formation of salt and water. Example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O) + Heat.

Examples of Neutralization in Daily Life-

Indigestion- Hydrochloric acid is found in our stomach. This acid helps in the digestion of food, but when there is an excess amount of acid in the stomach, it leads to indigestion. To relieve indigestion, we use an antacid like milk of magnesia, which contains magnesium hydroxide. It neutralizes the effect of the excess acid.

Ant Sting- When an ant stings, it injects an acidic fluid into the skin. The effect of the sting can be neutralized by applying a moist solution of baking soda (sodium bicarbonate) or calamine lotion, which contains zinc carbonate.

Soil Treatment- Excessive use of chemical fertilizers makes the soil acidic. When the soil becomes highly acidic, it is treated with alkalis like quicklime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is alkaline, organic substances are added. Organic substances (compost) neutralize the acid in the soil, which neutralizes its alkaline nature.

Factory Waste- Factory waste contains acidic substances, and before discharging it into reservoirs and rivers, alkaline substances are added to neutralize it.